Intel 386 manual Electromagnetic Interference CROSS-TALK

PHYSICAL DESIGN AND DEBUGGING

frequency design, including interference. In general, interference occurs when electrical activity in one conductor causes transient voltage to appear in another conductor. Two main factors increase the interference in any circuit:

1.Variation of current and voltage ,in the lines causes frequency interference. This interference increases with frequency.

2.Coupling occurs when conductors are in close proximity.

Two types of interference are observed in high frequency circuits:

1.Electromagnetic Interference (EMI)

2.Electrostatic Interference (ESI)

.11.4.3.1 ELECTROMAGNETIC INTERFERENCE (CROSS-TALK)

Cross-talk is a problem at high operating frequencies: when operating frequency increases, signal wavelength becomes comparable to the length of some of the intercon- nections on .the PC board. Cross-talk is a phenomenon of a signal in one trace which induces another similar signal in an adjacent trace. There are two types of couplings between parallel traces which determine the amount of cross-talk in a circuit. These are called inductive coupling and radiative coupling.

Inductive coupling occurs when current in one trace produces current in a parallel trace. This current decreases with increasing distance from the source. Hence, closely spaced wires or traces will incur the greatest degree of inductive coupling. Both the traces in this case act like normal conductors.

Radiative coupling occurs when two parallel traces act as a dipole antenna which radi- ates signals that parallel wires can pick up. This results in the corruption of the signal that is already present in the trace. The intensity of this type of coupling is directly proportional to the current present in the trace. However, it is inversely proportional to the distance between the radiating source and the receiver.

11.4.3.2 MINIMIZING CROSS-TALK

When laying out a board for a high frequency processor-based system, several guidelines should be followed to minimize cross-talk.

One source of cross-talk is the presence of a common impedance path. Figure 11-22 shows a typical layout which does not have the same earth ground and signal ground.

To reduce cross-talk, it is necessary to minimize the common impedance paths, which are Z2, Z3 and Z4 shown primarily as ground impedances. During current switching, the ground line voltage drops causing noise emission. By enlarging the ground conductor

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